U.S. patent number 5,834,113 [Application Number 08/472,404] was granted by the patent office on 1998-11-10 for self-reinforced ultra-high molecular weight polyethylene composite medical implants.
This patent grant is currently assigned to Poly-Med, Inc. Invention is credited to Meng Deng, Shalaby W. Shalaby.
United States Patent |
5,834,113 |
Shalaby , et al. |
November 10, 1998 |
Self-reinforced ultra-high molecular weight polyethylene composite
medical implants
Abstract
The invention provides composites of ultra-high molecular weight
polyethylene reinforced with ultra-high molecular weight
polyethylene anisotropic reinforcement of high strength and
modulus. The composites have superior mechanical properties
relative to non-filled ultra-high molecular weight polyethylene,
including higher strength, impact strength, increased creep
resistance, and improved modulus. The composites may be sterilized
for biomedical use, using gamma radiation and other techniques.
Further, the composites are resistant to the effect of body fluids
and have lower creep rates so that they will provide implant life.
The composites may be cross-linked by exposure to an acetylene
environment. Also, the composites find use in other high strength,
high impact applications such as sports equipment.
Inventors: |
Shalaby; Shalaby W. (Anderson,
SC), Deng; Meng (Clemson, SC) |
Assignee: |
Poly-Med, Inc (N/A)
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Family
ID: |
22331502 |
Appl.
No.: |
08/472,404 |
Filed: |
June 7, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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472404 |
Jun 7, 1995 |
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110155 |
Aug 20, 1993 |
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Current U.S.
Class: |
428/364; 428/373;
428/902 |
Current CPC
Class: |
B29C
70/06 (20130101); A61L 27/48 (20130101); C08L
23/06 (20130101); B29C 70/04 (20130101); B29C
70/465 (20130101); B29C 70/50 (20130101); B29C
67/24 (20130101); A61L 31/129 (20130101); A61L
27/48 (20130101); C08L 23/06 (20130101); A61L
31/129 (20130101); C08L 23/06 (20130101); C08L
23/06 (20130101); C08L 2666/04 (20130101); Y10S
428/902 (20130101); Y10T 428/2913 (20150115); C08L
2203/02 (20130101); B29L 2031/7532 (20130101); C08L
2205/02 (20130101); Y10T 428/2973 (20150115); B29K
2105/24 (20130101); B29K 2023/0683 (20130101); Y10T
428/31855 (20150401); C08L 2205/16 (20130101); Y10T
428/2929 (20150115); B29K 2105/101 (20130101) |
Current International
Class: |
B29C
70/06 (20060101); B29C 67/24 (20060101); B29C
70/04 (20060101); B29C 70/46 (20060101); B29C
70/50 (20060101); A61L 27/48 (20060101); A61L
27/00 (20060101); A61L 31/12 (20060101); C08L
23/06 (20060101); C08L 23/00 (20060101); D02G
003/00 (); A61F 002/30 () |
Field of
Search: |
;428/364,373,902
;264/138,163,722,323 ;623/18,73 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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83101731 |
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May 1991 |
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EP |
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PCT/US92/10005 |
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Jun 1993 |
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WO |
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Other References
Stern, et al., "Wear properties of retrieved carbon-reinforced and
UHMW-PE tibial components," Ultra-High Molecular Weight
Polyethylene as Biomateral in Orthopedic Surgery, H.G. Willert,
G.H. Buchhorn, and P. Eyerer, eds., Hogrefe & Huber Publishers
(1991), pp. 258-261. .
Wright, et al., "Carbon fiber-reinforced UHMWPE for total joint
replacement components," Composites in BioMedical Engineering, 1st
International Conference 1985, pp. 21/1-21/4. .
Mead, et al., "The preparation and tensile properties of
polyethylene composites," J. Applied Polymer Science, 22:3249-65
(1978). .
Hirte, et al., "A one polymer composite polyethylene film: failure
morphology," Morphology of Polymers, Water de Guyter & Co.,
1986, pp. 527-539. .
Ishida, H. and Bussi, P., "Surface-induced crystallization in
ultrahigh modulus polyethylene fiber reinforced polyethylene
composites," Macromolecules, 24:3569-77 (1991). .
Deng, et al., "Thermal and thermo-oxidation properties of virgin
UHMW-PE," The 19th Annual Meeting of the Society for Biomaterials,
Apr. 28-May 2, 1993, Birmingham, Alabama. .
Ciferri, A. and Ward, I.M., "Ultra-High Modulus Polymers," Applied
Science Publishers, Ltd., England 1979, pp. 70-75. .
McKenna, et al., "Mechanical properties of some fibre reinforced
polymer composites after implantation as fracture fixation plates,"
Biomaterials 1980, vol. 1, IPC Business Press (1980), pp. 189-192.
.
Soltz, U. and Richter, H., "Investigation of Mechanical Behaviour
of Fibre-reinforced Materials for Endoprosthetic Devices,"
Biomaterials 1982, G.D. Winter, Gibbons, and H. Pienk, Jr., eds.,
(1982), pp. 33-38. .
Bradley, J.S. and Hastings, G.W., "Carbon Fibre-Reinforced Plastics
for Orthopaedic Implants," Mechanical Properties of Biomaterials,
G.W. Hastings and D.F. Williams, eds., John Wiley & Sons Ltd.
(1980), pp. 379-386. .
Grobbelaar, C.J., et al., "The Radiation Improvement of
Polyethylene Prostheses: A Preliminary Study," The Journal of Bone
and Joint Surgery, vol. 60, No. 3 (Aug. 1978), pp. 370-374. .
Wright, et al., "The effect of carbon fiber reinforcement on
contact area, contact pressure, and time-dependent deformation in
polyethylene tibial components," J. of Biomedical Materials &
Research, vol. 15, pp. 719-730 (1981). .
Connelly, et al., "Fatigue Crack Propagation behavior of Ultrahigh
Molecular Weight Polyethylene," J. of Orthopedic Research, vol. 2,
No. 2, Raven Press (1984), pp. 119-125. .
Deng, M. and Shalaby, W., "Determinants of Thermal Events in
Ultrahigh Molecular Weight Polyethylene," Polymer for Advanced
Technologies, vol. 4 (1993), pp. 43-46. .
Wright, T. M., et al., "Analysis of Surface Damage in Retrieved
Carbon Fiber-Reinforced and Plain Polyethylene Tibial Components
from Posterior Stabilized Total Knee Replacements," The Journal of
Bone and Joint Surgery, vol. 70-A, No. 9 (Oct. 1988), pp.
1312-1319..
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Primary Examiner: Weisberger; Richard
Attorney, Agent or Firm: Gregory; Leigh P.
Parent Case Text
This application is a continuation of application Ser. No.
08/472,404 filed Jun. 7, 1995 which is a divisional application of
application Ser. No. 08/110,155 filed Aug. 20, 1993, now abandoned.
Claims
What is claimed is:
1. A composite medical implant of high tensile and impact strength,
elastic modulus, and creep resistance, the medical implant
comprising:
a composite, said composite including:
an essentially non-oriented matrix of an ultra-high molecular
weight polyethylene polymer; and
a reinforcement of ultra-high molecular weight polyethylene
dispersed in and bonded to said matrix in sufficient amount to
increase the tensile and impact strength, elastic modulus, and
creep resistance of the composite above the tensile and impact
strength, elastic modulus, and creep resistance of ultra-high
molecular weight polyethylene of the matrix in non-reinforced form,
the reinforcement comprising up to about 12 percent by weight of
the composite, the reinforcement selected from the group consisting
of fibers, plies of fibers, and textile constructs;
wherein a unidirectionally reinforced segment of the composite
exhibits a transverse strength equal to or greater than the
strength of a non-reinforced ultra-high molecular weight
polyethylene matrix polymer of substantially similar
dimensions.
2. The medical implant of claim 1, wherein the reinforcement
comprises plies, each ply comprising fibers therein parallel to
each other, the plies oriented in a direction so that the fibers
within the plies are at a predetermined angle to an axis of the
medical implant.
3. The medical implant of claim 2, wherein the angle of orientation
of fibers within the plies is selected to counteract a major force
applied to the implant when the implant is in use in a body.
4. The medical implants of claims 1 or 3, wherein the implants are
sterilized.
5. The medical implant of claim 1, wherein at least a portion of
the ultra-high molecular weight polyethylene matrix polymer is
cross-linked.
6. The medical implant of claim 5, wherein the cross-linked
ultra-high molecular weight polyethylene polymer is produced by
diffusion of acetylene into the matrix and chemical reaction of the
acetylene with the matrix polymer.
7. The medical implant of claim 1, wherein the implant is shaped
for maxillo-facial application in a living body.
8. A composite of high tensile strength, elastic modulus, and creep
resistance, the composite shaped in the form of a medical implant
suitable for implantation into a living body, the composite
comprising:
an essentially non-oriented matrix of an ultra-high molecular
weight polyethylene polymer; and
a reinforcement of ultra-high molecular weight polyethylene
dispersed in and bonded to said matrix in sufficient amount to
increase the tensile and impact strength, elastic modulus, and
creep resistance of the composite above the tensile and impact
strength, elastic modulus, and creep resistance of non-reinforced
ultra-high molecular weight polyethylene matrix polymer, the
reinforcement comprising up to about 12 percent by weight of the
composite, the reinforcement selected from the group consisting of
fibers, plies of fibers, and textile constructs;
wherein a unidirectionally reinforced segment of the composite
exhibits a transverse strength equal to or greater than the
strength of non-reinforced ultra-high molecular weight polyethylene
matrix polymer of substantially similar dimensions.
9. The medical implant of claim 1 or claim 2, wherein the composite
comprises from about 3 wt. % to about 12 wt. % reinforcement, base
on the weight of the reinforcement and the matrix polymer.
10. The medical implant of claim 3, wherein the composite comprises
from about 3 wt. % to about 12 wt. % reinforcement, based on the
weight of the reinforcement and the matrix polymer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to novel composites of ultra-high molecular
weight polyethylene (UHMWPE) self-reinforced by high strength and
modulus fibers of UHMWPE, and methods for composite production and
sterilization for biomedical use. The composite has superior
mechanical properties which allows its use in applications where
high strength, creep resistance, and impact resistance are
important, such as sports equipment, protective equipment, medical
implants, external prostheses, medical appliance components, and
the like.
2. Description of the Related Art
A major application of ultra-high molecular weight polyethylene
(UHMWPE) in medical devices is its use as load-bearing components
of articulating joint prostheses, such as hip and knee prostheses.
As this type of surgery is applied to younger patients and a longer
implant-life span is expected, the failure of prostheses to
function, due to cold flow (creep) of UHMWPE, becomes important. In
the past, attempts were made to improve the long term performance
(wear and creep resistance) of UHMWPE without significant success.
Amongst the most-explored approaches to control the UHMWPE creep
was its reinforcement with carbon fibers. It was assumed that
increased creep resistance would also benefit the wear property of
the polymer. However, analytical and clinical studies of carbon
fiber reinforced UHMWPE composites, as used in components for total
joint replacement, have shown no advantage in using such composite
materials in place of non-filled UHMWPE. In fact, the wear
characteristics of the composite were inferior to those of the
non-filled polymer: both coefficient of friction and wear rate
increased. Fatigue crack propagation resistance was found to be
significantly worse in the composite than in UHMWPE. These results
were attributed to: (1) brittle nature of carbon fibers; (2)
difference in molecular structure between UHMWPE and carbon; (3)
existence of residual stress in the composite due to mismatch of
thermal expansion coefficients of carbon fiber and UHMWPE; (4) poor
bonding between the carbon fibers and the UHMWPE matrix; and (5)
the ductile nature of the matrix itself. As a result, the carbon
fiber reinforced UHNWPE orthopedic implants have been removed from
the market.
Mead and Porter describe composites prepared from solid state
extruded low molecular weight (58,000 or 18,400 number average
molecular weight) polyethylene fibers and polyethylene matrices
(both high and low density). W. T. MEAD AND R. S. PORTER, THE
PREPARATION AND TENSILE PROPERTIES OF POLYETHYLENE COMPOSITES, J.
APPLIED POLYMER SCIENCE, 22: 3249-3265 (1978). Harpell, Kavesh,
Palley, and Prevorsek made composites based on UHMWPE fibers (at
least of about 500,000 molecular weight) of high tenacity and
modulus, and lower molecular weight polyethylene matrices (low
density, high density, and linear low density polyethylene). G. A.
Harpell, S. Kavesh, I. Palley, and D. C. Prevorsek, Composite
Containing Polyolef in Fiber and Polyolein Polymer Matrix, European
Patent 83101731 (1983). High or ultra-high strength and-modulus
UHMWPE fibers have sufficient strength and stiffness for using as
reinforcement, and at the same time they possess a ductile nature.
However, a composite of an UHMWPE matrix reinforced by high
strength and modulus UHMWPE fibers has not heretofore been known in
the art. Constraints associated with the difficulty in melt
processing fibers having almost identical melting properties, with
matrices may have been the reason for no prior interest in this
system.
SUMMARY OF THE INVENTION
The invention provides a composite of ultra-high molecular weight
polyethylene (UHMWPE) that has enhanced strength, modulus, impact
strength, and creep resistance. The composite includes a polymeric
matrix of UHMWPE and a reinforcement, made of UHMWPE, distributed
within the matrix. The reinforcement may be selected from plies
made of parallel UHMWPE fibers, short portions of UHMWPE fibers,
mechanically anisotropic UHMWPE particulates, and the like, in a
sufficient amount to provide the desired improvement in strength,
modulus, and creep resistance.
The self-reinforced composites of the invention have several
surprising features. Among these are that, unlike prior art
teachings and practice, they can be cross-linked with high energy
radiation such as gamma radiation and electron beam radiation to
obtain improved strength and creep resistance. Further, they have
improved tensile strength in a direction transverse to fibrous
UHMWPE reinforcement, relative to non-filled UHMWPE, when usually a
non-filled polymer is stronger than its composites in this
direction. Additionally, the composites of the invention have
improved impact strength, although they are filled with a high
modulus material. Normally, such impact strength improvement is
only seen when low modulus "rubbery" fillers are used. Also, while
other composites normally achieve optimal physical properties at
filler loadings of 30 wt. % or more, the composites of the
invention require much less filler. The optimum range of filler
addition is from about 3 to about 12 wt. % with physical properties
decreasing as filler exceeds about 12 wt. %. Finally, the use of
UHMWPE as a reinforcement material is necessary. When other filler
materials were used, the enhancement of UHMWPE physical properties
was insignificant compared to that obtained with UHMWPE
reinforcement.
The composites may be sterilized using a gamma irradiation or
electron beam process without the decline of physical properties
experienced by non-reinforced UHMWPE and therefore may be
fabricated into superior medical implants, including those implants
requiring high strength, such as load bearing orthopedic implants.
The composites are, of course, also useful in other medical implant
applications including bone screws, bone plates, skull plates,
cranial devices, fracture fixation devices, intramedullary nails,
maxillo-facial implants, and the like. Further, the composites do
not lose their enhanced mechanical properties to any significant
extent upon prolonged exposure to saline solution or body fluids,
when compared to UHMWPE constructs of commercially pure
polymer.
Due to the strength and impact resistance of the composites of the
invention, they are well-suited to other applications demanding
such materials, for example sports equipment, including, but not
limited to, skis, ski poles, goggle frames, protective helmets,
mountaineer's equipment, and the like, as well as specialized
applications in aerospace and the like. Also, the composites may be
used for external medical support such as braces of all kinds,
crutches, splints, artificial limbs, and the like.
The invention also provides yet further strength-enhanced
composites of UHMWPE reinforced with UHMWPE components, wherein at
least some of the UHMWPE polymer of the composite matrix is
cross-linked. This cross-linking may be carried out by exposure of
the composite to acetylene gas under suitable conditions to allow
the acetylene to penetrate the composite by diffusion and
chemically react to produce cross-linked molecules.
Further, the cross-linking may be achieved by exposure to high
energy radiation such as gamma or electron beam radiation.
Surprisingly, irradiation by high energy produces cross-linking in
such excess over bond scission or lysis that the net effect is a
composite of enhanced tensile strength, elastic modulus, creep
resistance, and impact strength. Further, since these irradiation
treatments also sterilize the composites, they are highly suitable
for use with medical implants, providing both sterilization and
cross-linking in one step.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the present invention can be obtained
when the following detailed description of the preferred embodiment
is considered in conjunction with the following drawings, in
which:
FIG. 1 is a graphic depiction of tensile yield strength versus
percent fibers for composites of the invention.
FIG. 2 is a graphic representation of tensile yield strength versus
percent fibers for unidirectional composites of the invention.
FIG. 3 is a graphic representation of the tensile modulus of
ultra-high molecular weight polyethylene as compared to composites
according to the invention.
FIG. 4 is a graphic depiction of tensile yield strength of
cross-ply composites versus percent fiber for composites according
to the invention.
FIG. 5 is a graphic depiction of tensile creep over time at room
temperature under a 5 MPa stress for-ultra-high molecular
polyethylene as a control and composites according to the
invention.
FIG. 6 is a graphic depiction of the effect of a 2.50 Mrad dose of
gamma radiation on the tensile stress of a control and ultra-high
molecular weight polyethylene composites of the invention in an
environment of air, nitrogen, or acetylene.
FIG. 7 is a graphic depiction of the tensile moduli of
gamma-irradiated unidirectional composites as compared to a control
of ultra-high molecular weight polyethylene.
FIG. 8 is a graphic depiction of the tensile yield strength of
gamma-irradiated unidirectional composites along the fiber axis as
compared to a control of ultra-high molecular weight
polyethylene.
FIG. 9 is a graphic depiction of the tensile modula of
gamma-irradiated unidirectional composites taken along the fiber
axis, as compared to a control of UHMWPE.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention composites include two necessary components: a matrix
of ultra-high molecular weight polyethylene (UHMWPE) and a
self-reinforcement made of UHMWPE, distributed within the
matrix.
Preferably, the UHMWPE is in the form of fibers. These fibers may
be present in the form of a ply, which is a sheet of fibers,
wherein each fiber is aligned in a direction parallel to the other
fibers. Preferably, the UHMWPE fibers have a molecular weight of at
least about 1 million daltons, and up to about 5 million daltons.
Commercially, such fibers may be obtained as SPECTRA 1000 and
SPECTRA 900 from Allied Signal Corp. The peak melting temperature
of such a fiber (measured at 10.degree. C./min. using Thermo
Analyst 2000 of TA Instruments) is at least about 140.degree.
C.
In preparing composites, the plies may be arranged so that the
fibers are aligned at an angle ranging from 0.degree. to 90.degree.
to an axis of the composite to be produced. These angles may be
selected depending upon the properties that are required in the
composite.
The highly oriented fibers display higher modulus and impact
resistance than the matrix. The contribution of the fibers to the
composite's mechanical properties depends in part on the direction
of orientation of the fibers relative to an applied load. Since
fibers display highest physical properties in their longitudinal
direction, it is preferred to orient fibers so that the applied
force acts against a longitudinal direction of the fibers. Thus, in
considering a composite with an axis along which an applied load
will act, the fibers may optionally be aligned parallel to the axis
("longitudinally"), at 90.degree. to the axis ("transversely"), or
at an angle between 0.degree. and 90.degree. to the axis
("cross-plied"). Of these orientations, the longitudinal is the
most preferred and the transverse is the least preferred.
While it is normally found that other composites are weaker in
tension under transverse applied forces than their matrix polymers,
this is surprisingly not the case for the composites of the
invention. It is theorized, without being bound, that interfacial
bonding or molecular interpenetration between the UHMWPE matrix and
UHMWPE reinforcement is unusually strong so that there is no
resultant weakness at the polymer-reinforcement interface found in
other composites.
While it is preferred that the fibers be present in the form of
plies to produce composites of controlled physical properties,
other types of UHMWPE reinforcement may also be used. These include
fibers (not in plies), anisotropic UHMWPE particulates, short
portions of UHMWPE fibers, textile constructs of UHMWPE fibers such
as fabric or tape, and the like.
The matrix polymer may include any UHMWPE that has a molecular
weight of preferably at least about 1 million daltons and up to
about 5 million daltons. Examples of such polymers may be
commercially obtained as GUR405 and GUR402 from Hoechst Celanese
Corporation, as fine powders with a peak melting temperature of
above 140.degree. C. (measured in the same manner as for the
fibers, explained above). Upon heating UHMWPE powder above its
melting temperature, and cooling to room temperature and then
reheating, melting occurs at a lower temperature, about 10.degree.
C. lower than the melting temperature of the fine powder. This
thermal behavior of UHMWPE is critical in the present
invention.
Preferably, the UHMWPE is processed in an inert environment since
it undergoes accelerated oxidation at elevated temperatures. It
should be noted that oxygen can form "transient cross-linking" in
UHMWPE. Since such cross-linking is undesirable, it is preferred to
perform the steps requiring heating and melting of UHMWPE in an
inert environment substantially free of oxygen.
When an UHMWPE matrix resin is molded into a form of sheets or
films for preparing the composite, the temperature is preferably
kept below 200.degree. C. and this temperature is preferably
applied for a time less than about 5 minutes, depending upon sheet
geometry. Under atmospheric pressure conditions, processing
temperature should preferably not exceed 235.degree. C., which is
close to the oxidation temperature of UHMWPE, and holding time
should be minimized. Higher temperatures may be used if processing
is carried out under an inert gas environment. These higher
temperatures also facilitate the fusion of the composites under
laminating conditions.
Generally, composites of the invention are produced by filling a
matrix polymer with fibers in the range from about 1 to about 20
wt. %, based upon the weight of the reinforcement and the matrix
polymer. Preferably, the composites contain from about 3 to about
12 wt. % reinforcement. This is in stark contrast to the art which
generally teaches loading with reinforcement at levels of 30% or
more. Here, contrary to the art's general teaching, it has been
found that the UHNWPE composites of the invention with the best
physical properties have from 3 to 12 wt. % reinforcement. Indeed,
using more than about 12 wt. % reinforcement causes a decline in
properties of the composites of the invention, although they are
still useful with up to about 30 wt. % reinforcement. When other
organic fibers, like nylon-6 and polyethylene terephthalate (PET)
fibers, have been incorporated into UHMWPE, at 3-10% fiber loading,
the resultant composites have shown very limited improvements in
UHMWPE properties. Further, the composite physical properties were
found to worsen after aging in 37.degree. C. water.
In general, layers or mixtures of polymer matrix and reinforcement
are processed at about 130.degree. C., under moderate pressure
ranging from about 4 to about 12 MPa to produce the composite of
the invention.
To reduce the melting temperature of the UHMWPE matrix, virgin
UHMWPE powder is first melted and pressed into sheets or films in a
mold, such as a rectangular metal frame with an upper and lower
steel pressing plate. Pressing is carried out under conditions that
cause the UHMWPE powder to melt and form a sheet or film of UHMWPE.
Such conditions will vary depending upon film thickness and UHMWPE
melting point. For a 0.025-0.035 inches thick film, this pressing
is preferably carried out at a temperature of from about
150.degree. to about 220.degree. C., more preferably 175.degree.
C.; and preferably under a pressure of from about 2 to about 13
MPa, more preferably about 7 MPa, for about 3 minutes. Clearly,
temperature and time are dependent on sheet geometry, especially
thickness.
As an alternative, sheets or films of the matrix polymer may be
produced by melt crystallizing UHMWPE powder under pressure of
preferably less than about 2 MPa. The resultant sheets or films are
porous, which facilitates melt diffusion when the composite is
assembled.
When laminating the composites, it is preferable to apply pressure
when the temperature reaches the required level. During lamination,
the sheets of polymeric matrix and reinforcement are heated up to a
temperature and for a time so that the film is able to melt at
least partially to then coat the reinforcement so that a unitary
solid is produced upon cooling. Typically, for a 6 mm thick
composite, the polymer film-reinforcement construct is heated to
about 135.degree. C. and subjected to pressures ranging up to about
6 MPa for about 30 minutes. During the lamination of the composite,
when plies or longitudinal fibers of UHMWPE form the reinforcement,
it is preferable to apply axial tension to the fibers to avoid
molecular chain relaxation at high temperatures and thereby prevent
non-uniform contraction of the fibers. Preferably, such composites
should be removed from the mold when the mold temperature has
dropped to below 70.degree. C., more preferably below 40.degree.
C.
Composites according to the invention may be sterilized under an
ethylene oxide atmosphere if no cross-linking reactions are
desired, or in the alternative, with high energy radiation,
preferably gamma, x-ray, or electron beam radiation. As explained
above, such high energy radiation also cross-links the UHMWPE
composites of the invention and unexpectedly enhances certain
physical properties. Preferably, this radiation is applied when the
composite is in an acetylene-containing environment to enhance
cross-linking. Therefore, the composites are eminently suitable for
use in the fabrication of constructs for high strength, high impact
use, including sports equipment, protective equipment, and the
like. UHMWPE is biocompatible and provides high strength and high
modulus. Composites according to the invention have enhanced
physical properties relative to non-reinforced UHMWPE and do not
suffer significant deterioration in physical properties when
subjected to saline solutions at 37.degree. C. for prolonged
periods of time. Further, composites of the invention have
increased creep resistance, a critical shortcoming in current
UHMWPE constructs. Thus, the composites of the invention are highly
suitable for use in medical implants, including load-bearing
prosthetic implants, and the like.
As used in the specification and claims, the term "medical
implants" includes all those devices that are implantable into
living bodies to provide for augmentation, support, repair, or
reconstruction of body tissue, especially bone.
In a further application, the composites of the invention can be
used to fabricate components of medical appliances. The term
"medical appliances" refers to externally worn or used devices, or
components thereof, that brace or support a living body, including,
but not limited to, knee braces, back braces, crutches, cervical
collars, and the like.
When composites according to the invention are subjected to an
acetylene gas environment, under conditions that will allow
diffusion of the acetylene into the composite, the acetylene reacts
with at least some of the UHMWPE to produce cross-linking. Thus, by
judicious selection of acetylene soaking time, temperature, and
pressure, a desired level of crystallinity may be obtained in a
composite. Such cross-linking further enhances the strength and
creep resistance of the composite.
The following examples are illustrative of the invention and do not
limit the scope of the invention as described above and claimed
below.
EXAMPLE 1
The materials used in Example 1 are UHMWPE resin as matrix (trade
name GUR405 from Hoechst Celanese Corporation, U.S.A.) supplied as
a virgin fine powder, and high strength and modulus UHMWPE fiber as
reinforcement (trade name SPECTRA 1000 from Allied Signal
Corporation, U.S.A.). The UHMWPE powder cannot be used directly in
making composites because its melting point temperature is very
close to that of the fiber so that fibers might also melt. To
reduce the intended matrix polymer's melting temperature, the
UHMWPE powder was first melted and the molten polymer then pressed
into sheets in a metal frame between two stainless steel plates at
a temperature of about 180.degree. C. and a pressure of about 7 MPa
for 3 minutes. The resulting sheet dimensions were 12 cm.times.9
cm.times.0.025 cm (weighing about 3 grams) and 12 cm.times.9
cm.times.0.035 cm (weighing about 4 grams).
SPECTRA 1000 fibers were wound twice onto a metal frame, which has
a thickness less than that of the expected composite, to obtain
four layers of fibers. Five UHNWPE sheets were arranged, one each
of thickness 0.035 cm on top and at the bottom, and three of
thickness 0.025 cm between each fiber layer. The resulting
construct polymer-fiber was melted and molded into a composite by
pressing between two stainless plates at the temperature of about
130.degree. C. and pressure of about 6 MPa for about 10 minutes
then cooled under pressure in a CARVER Laboratory Press (Model C).
The composite was taken out of the mold after the temperature
dropped below 60.degree. C. (If no forced cooling is used, then it
takes about 3 hours for the temperature to drop below 60.degree.
C.) The composite, which can be represented by a laminate code of
[/0/0/0/0/0/].sub.t (where "0" represents the angle of orientation
of a layer of fibers and "/" a layer of matrix), had a dimension of
about 12 cm.times.9 cm.times.0.15 cm, and a fiber loading of 4% by
weight, with a very fine surface finish, which is dependent on the
roughness of the pressing metal plates used.
EXAMPLE 2
Following the same procedure as in Example 1, a cross-ply composite
[/0/90/90/0/].sub.t with a fiber loading of 4% was obtained by
winding the fiber onto the metal frame so that two layers are in a
90.degree. direction and two layers in 0.degree. direction relative
to an axis of the composite.
EXAMPLE 3
UHMWPE sheets were obtained by first melt-processing the UHMWPE
powder (of Example 1) without applied pressure between two
stainless plates at about 180.degree. C. for about 3 minutes in a
Carver Laboratory Press. The resulting polymer sheets were porous.
When making composites, these porous sheets diffuse and bond
together more easily than those of Examples 1 and 2. Thus, it is
preferred to make polymer sheets this way. The other procedures for
making composites are the same as in Examples 1 and 2. It is
expected that the bonding of polymers to fibers according to this
example would be better than those in Examples 1 and 2, although
tests were not performed to characterize interfacial strength
between matrices and fibers, due to the difficulty of measuring
this property.
Films made under less than 2 MPa pressure will provide a facile
melt diffusion during assembly and hence maximize adhesion between
fiber and polymer. This maximizing of adhesion beneficially affects
physical properties. Thus, contrary to expectation, tensile
strength in the transverse direction is higher than for unfilled
UHMWPE, a characteristic which is not found in other
composites.
Composites with various stacking sequence (fiber orientation) and
fiber loadings were made for mechanical testing. Dogbone tension
specimens were cut from the composite sheets (in both longitudinal
and transverse directions) using a metal die and tensile strength
was tested on the specimens on an INSTRON universal mechanical
tester. The results (average of at least 5 specimens for each case)
are summarized in Table 1. Also included in Table 1 are the results
for the non-reinforced UHMWPE samples (made and tested under the
same conditions as for the composite), for the purpose of
comparison. Compared with the non-reinforced UHMWPE, the composites
have greater yield strength and relative modulus in the
longitudinal direction and showed almost no change in transverse
direction. Small increases of strength and stiffness in the
transverse direction indicate a good interfacial bonding. Cross-ply
laminates also showed superior properties.
TABLE 1 ______________________________________ Tensile Property of
GUR405 and Its Composites (V.sub.f = .about.5.5 wt. %) Yield
Failure Sample Stress, MPA Stress, MPa Modulus, MPa
______________________________________ GUR405 sheet 24.7 47.9 578
[/0/0/0/0/].sub.t 117.1 40.2 2652 [/90/90/90/90/].sub.t 24.7 35.9
694 [0/90/90/0/].sub.s 56.7 41.5 1524
______________________________________
EXAMPLE 4
Following the same procedure as in Example 3, a composite with a
thickness of greater than 5 mm was obtained by using 17 layers of
processed UHMWPE sheets and 16 layers of fibers. The typical
stacking sequence was [0/90.sub.2 /0.sub.2 /90.sub.2 /0].sub.s. The
composites had a very fine surface finish and a fiber loading of 5%
by weight.
EXAMPLE 5
Following the same procedure as in Example 3, composites with
stacking sequences of [/0/0/0/0/].sub.t and [/90/90/90/90/].sub.t
were assembled. Each composite had a fiber loading of 5 wt. %.
Gamma irradiation of the composites was conducted using a dose of
2.50 Mrads in three different gas environments, namely, air,
nitrogen (practically pure), and acetylene. For each condition, 5
specimens were used.
Two ground-jointed separatory funnels, each with 15 specimens
inside for each case, were sterilized in the nitrogen and acetylene
environments, respectively. The purge cycles were as follows:
evacuate separatory funnel containing samples; purge nitrogen or
acetylene; evacuate separatory funnel again; purge nitrogen or
acetylene once more.
The duration for each step was about 15 minutes. A total of four
cycles was used. Finally, the separatory funnel was sealed and no
gas exchange occurred. For acetylene gas the pressure inside
separatory funnel had a gauge reading of about 2.0 psi, and for
nitrogen gas the pressure inside was just above atmospheric
pressure. Irradiation was done normally after at least 3 days so
that the gases could diffuse into the samples.
Gamma irradiation was completed in a gamma irradiator using .sup.60
Co isotopes as a source. All irradiations were done at room
temperature. Following gamma irradiation, a tensile test was run on
the specimens on an INSTRON universal mechanical tester (Model
1125) at room temperature using a loading velocity of 20 mm/min.
Gauge length was 20 mm and the entire testing was run under
computer control. All the mechanical tests were run within three
weeks after exposure to gamma radiation.
The results are illustrated in FIGS. 6-9 from which it can be
understood that gamma irradiation sterilization in
acetylene-containing environment is preferred.
EXAMPLE 6
Following the same procedure as in Example 3, composites with
stacking sequences of [/0/0/0/0/].sub.t and [/0/90/90/0/].sub.t
were assembled. Each composite had a fiber loading of 5 wt. %. A
constant load creep test was run on the composites and GUR405
polymer as a control. Table 2 lists the compliances of the 24-hour
room temperature creep tests. The superior creep-resistance of
composites over the non-reinforced UHMWPE control can be seen from
this table or from the compliance versus time plot in FIG. 5.
TABLE 2 ______________________________________ Creep Compliance
after 24 Hours at 5 MPa Sample Compliance (1/GPa)
______________________________________ GUR405 sheet 2.89
[0/0/0/0/].sub.t 1.05 [0/90/90/0/].sub.t 1.14
______________________________________
EXAMPLE 7
Following the same procedure as in Example 3, cross-ply composites
were assembled. Each composite had a fiber loading of 4.2 wt. %.
Composites were gamma sterilized at a dose of 2.50 Mrads in air.
Double-razor notch izod impact tests were performed on the
composites, and also on plain UHMWPE for comparison. Table 3 lists
the results.
TABLE 3 ______________________________________ Double-Razor Notch
Load Izod Impact Test Results Strength Notch/Fiber (Ft- Sample
.gamma.-Sterilized Orientation 1b/in.sup.2)
______________________________________ GUR405 sheet No N/A 55 .+-.
0 Yes N/A 44 .+-. 1 [//0/90/0/90/0/90/0/90/0/].sub.s Yes Notch
between 53 .+-. 2 fibers Yes Notch along fibers 47 .+-. 6
[//90/0/90/0/90/0/90/0/].sub.s Yes Notch between 59 .+-. 2 fibers
Yes Notch along fibers 51 .+-. 2
______________________________________
From this data, it is apparent that unfilled UHMWPE loses about 20%
of strength on exposure to gamma radiation. However, such strength
loss is surprisingly not found in the composites which instead show
strength improvement by about 7 to about 30% due to reinforcement.
Also, while filling with rubbery, low modulus fillers is known to
improve impact strength, here surprisingly, composites filled with
high modulus fibers have improved impact strength.
Although the invention has been described with reference to its
preferred embodiments, those of ordinary skill in the art may, upon
reading this disclosure, appreciate changes and modifications which
may be made and which do not depart from the scope and spirit of
the invention as described above and claimed below.
* * * * *